Since Russia's annexation of Crimea in March, NATO has been publicly refocusing on its old Cold War foe Moscow. The threats it now believes it faces, however, are distinctly different to those of the latter half of the 20th century.

The West then was defending against the risk of Soviet armor pouring across the North German plain. Now, officials and experts say, it is “ambiguous warfare” that is focusing minds within NATO, the North Atlantic Treaty Organization.

Confrontations are viewed as more likely to start with cyber attacks or covert action to stir up Russian minorities in Europe's east than from any overt aggression.

So as NATO prepares for its summit on Sept. 4 and 5 in Wales, it is having to come to grips with relatively new threats to test Article 5 of its treaty. That essentially says that an attack on one NATO state is an attack on all.

Since NATO's post-Cold War expansion that has meant protecting eastern members including the Baltic states. Lithuania, Latvia and Estonia all have considerable Russian minorities while Poland and others worry Russia still views them as within its sphere of influence.

High-profile troop, aircraft and ship deployments and exercises have been designed to send the message that the United States and its allies would react with force to any attack on its territory.

A less conventional attack, however, could be harder to defend against. For example, without firm proof that Moscow was behind a cyber attack or covert action, deciding whether to invoke Article 5 would be very difficult.

“This is new territory but it's something that is going to have to be discussed,” said Janine Davidson, U.S. deputy assistant secretary of defense for plans from 2009-12 and now senior fellow at the Council on Foreign Relations. “It is very difficult to know how to react to it. It will have to be very much on a case-by-case basis.”

Events in non-NATO member Ukraine, senior officials say, could be a sign of how complicated things might get.

Ukrainian and Western officials accuse Moscow of arming and training separatist rebels who have now been fighting the Ukrainian military for months.

Some NATO officials privately and publicly worry the same could happen in Russian-speaking regions of the Baltic states of Lithuania, Latvia and

Similarly, in 2007, a series of crippling cyber attacks paralyzed much of Estonia in an apparent response to a dispute over the movement of a Soviet-era war memorial. Most Western experts suspected the Kremlin was responsible.

Russia has denied involvement with rebels in Ukraine and says the 2007 cyber attacks were simply by “patriotic (independent) hackers.”

But it leaves NATO wondering how to react.

“We need to mature the way we think about cyber, the way we think about irregular warfare,” U.S. Air Force General Philip Breedlove, NATO's supreme allied commander, was quoted as saying by the U.S. military Stripes newspaper.

Between diplomacy and war

The Cold War was fought through espionage and proxy wars across much of Africa, Asia and Latin America. Both sides knew that any serious military incursion into Soviet or NATO territory would almost inevitably spark nuclear war.

The difference now, strategists say, is the perceived greater potential for Russian interference in the new NATO member states it dominated for decades.

NATO does have its own unconventional capabilities. Experienced in operating with tribal and militant groups in Afghanistan, the Middle East and Africa, U.S. special forces and intelligence personnel could theoretically stir up trouble in Russia.

Agencies such as the U.S. National Security Agency and Britain's GCHQ could also wreak cyber havoc on Russian telecoms and other systems.

For now, however, there is little precedent for such decisions.

“What they (NATO) don't have at this stage is any kind of doctrine for using them in situations short of outright war,” says John Bassett, a former GCHQ official now an associate at Oxford University.

“There's been a lack of willingness to focus on the area beyond diplomacy but below the threshold of traditional military conflict and there is still a very long way to go.”

Some strategists suggest NATO is not currently up to dealing with the situation.

“A Russian unconventional attack, using asymmetric tactics designed to slip below NATO's response threshold, would be particularly difficult to counter,” said a report last month from Britain's Parliamentary defense select committee.

It added: “The challenges, which NATO faces in deterring, or mounting an adequate response to, such an attack poses a fundamental risk to NATO's credibility.”

Recently, however, senior officials have begun quietly laying out some of NATO's new red lines.

Breedlove, the NATO commander, said last weekend the Alliance would react militarily if Russian troops infiltrating a member state territory in the way the West believed they did in Crimea.

That intervention, Western officials say, was rather more obvious than more recent events in eastern Ukraine. Russian-speaking troops in uniform but without insignia took up checkpoints across the peninsula and surrounded Ukrainian military bases. Moscow then formally annexed territory.

The United States has also publicly stated that it might react with conventional military force to a cyber attack that took lives or inflicted serious material damage.